In general, commercial ceramic arc discharge vessels used in high intensity discharge (HID) lamps are comprised of a polycrystalline alumina ceramic, which may contain one or more additives to control grain growth. As a first step, alumina powder is mixed with a binder material such as a wax or thermoplastic and then formed into the desired shape by isostatic pressing, extrusion, or injection molding. The binder materials help the molded alumina piece retain its shape while the piece is in its “green state,” i.e., prior to binder removal and sintering. The binder is later removed when the pieces are fired.
Since the arc discharge vessels are fabricated from two or more pieces, it is necessary to form hermetic seals at the interfaces between the pieces, which hermetic seals are capable of withstanding the high stresses, temperatures and corrosive chemicals present in an operating arc discharge vessel. The conventional method of assembling ceramic arc discharge vessel pieces involves several assembly and pre-sintering steps in which the pieces are aligned and sealed together by means of interference fits. The interference fits result from the differential shrinkage of the pieces during firing. In each of the assembly and pre-sintering steps, there exists an opportunity for misalignment or other errors to occur. Minimizing the number of firing cycles can improve the efficiency of the arc discharge vessel production process. Furthermore, the practice of using interference fits to form the hermetic seals requires a high degree of control over dimensional tolerances and the shrinkage of the ceramic pieces during firing.
Several methods are available for solving the disadvantages enumerated above. One method, which is preferred, comprises joining two body halves in the green state. The method includes applying heat to the surfaces to be joined to cause a localized melting of the binder. The surfaces are then brought together and joined by applying compression. Another method is disclosed in U.S. Pat. No. 6,620,272, which is incorporated herein by reference, and comprises a method for making a ceramic body wherein the ceramic components are joined in their green state. The method includes applying heat to the surfaces to be joined to cause a localized melting of the binder. The surfaces are then brought together and joined by alternately applying compression and stretching. These methods are particularly advantageous for forming unitary ceramic arc discharge vessel bodies for high intensity discharge (HID) lighting applications; however, it has been found that these processes, while extremely efficient for ordinary ceramic arc discharge vessels, have a potential for axial misalignment during the joining process which may be detrimental to certain types of ceramic metal halide lamps, particularly those that use acoustically stabilized arcs.
Misalignment of the two halves of the discharge vessel during the joining process can be reduced by careful adjustment of the joining equipment to insure that the two carriers of the ceramic parts are as close to coaxial as possible. However, as described above, even careful alignment may not be sufficient for certain types of arc discharge vessels, particularly ones that require smaller diameters. In these case, it may become necessary to resort to visual inspection to remove misaligned parts. Thus, for such high efficiency lamp designs, it would be advantageous to have a method that would minimize or eliminate altogether axial misalignment of the ceramic parts that form the discharge cavity.